What is a characteristic of Indirect ELISA?

• A. It includes an additional amplification step to detect antibodies in a sample.
• B. It uses a conjugated primary antibody for detection.
• C. It does not involve any secondary antibodies.
• D. It measures enzyme activity directly without antigen binding.

Answer: (A)

Indirect ELISA detects antibodies in a sample by using a labeled secondary antibody to reveal the presence of the primary antibody bound to antigen. The key idea is that after the antigen on the plate captures specific antibodies from the sample, a secondary antibody that recognizes those antibodies is added and is conjugated to an enzyme. This secondary antibody binding creates a larger signal because multiple secondary antibodies can attach to a single primary antibody, amplifying the detectable signal when the substrate is converted by the enzyme. This amplification step is what makes indirect ELISA sensitive for detecting antibodies.

That’s why the statement about an additional amplification step is correct. It wouldn’t be correct to say a conjugated primary antibody is used (that describes direct ELISA), or that no secondary antibodies are involved, or that the enzyme activity is measured without any antigen-antibody binding, since the signal in indirect ELISA depends on the antigen–antibody interaction followed by enzymatic detection via the secondary antibody.